STEAM TRAP


STEAM TRAP

In order to receive the greatest return from the cost of generating and using steam as a heating and process medium, a good working knowledge of a steam traps purpose, operation, and application is essential. The information contained in this chapter is designed to aid you in effectively selecting steam traps since most problems encountered by users can be prevented by installing the correct type of trap with an approved hook up. 

Why a steam trap? 

Steam generated by a boiler contains heat units, British Thermal Unit, which are released in the heating, process applications, and by pipe radiation loss, and in doing so the steam returns to its water, or condensate, state. If condensate is not drained immediately or “trapped” from the system it reduces operating efficiency by slowing the heat transfer process and can cause physical damage through the phenomenon known as “Water Hammer.”

The velocity of the steam passing over it sweeps a condensate accumulating on the bottom of a horizontal pipe along. As the velocity increases, the condensate can form a solid slug of incompressible water, which, when it is suddenly stopped by a pipe bend, fitting, or valve, produces a shock wave of tremendous momentary force, which can cause great physical damage. Since water hammer is a function of steam velocity, it is as dangerous in low pressure systems as it is in high pressure systems and the piping must be designed to drain properly avoiding water pockets. 

As trap is therefore required at all locations where condensate is formed and collects, principally- 

Steam Distribution Mains:

(a)  At elevation changes such as risers and expansion loops.

(b)  At all low points and at intervals of 60 to 150 metre. On long horizontal runs.

(c)  A head of all possible “dead-ended” areas such as shutoff valves, pressure and temperature control valves, and at ends of mains. 

Steam Tracing Service:  

(a)  On traced and jacked piping and valves.

(b)  For freeze protection of instruments and equipment. 

Steam Operated Equipment: 

(a)  Ahead of humidifiers, pumps, turbines, etc.

(b)  Draining directly below heat exchangers, coils, unit heaters, cooking kettles,          driers, etc. 

Air Venting 

The air and other non-condensable gasses such as carbon dioxide and carbon monoxide must also be rapidly purged from the steam system by the trap or by an auxiliary air vent for four important reasons. 

(1)  On start-up, steam enters the system only as fast as the air is vented.

(2)  An air-steam mixture has a temperature well below steam temperature lowering      the heat transferred.

(3)  Air is an insulator and clings to the surface of the pipe or vessel causing slow and   uneven heat transfer.

(4)  Dissolved in condensate, these non-condensable gasses form corrosive carbonic     acid, which attacks system. 

The steam trap therefore is an automatic drain valve which must sense the difference between steam and condensate, operate under varying inlet and back pressures, changing condensate loads and must also release non-condensable gases while not wasting any steam.

 Trap Discharge Temperature  

The temperature at which the condensate is discharged by the trap is important in maintaining energy efficiency. While most applications require that the condensate be discharged at close to steam temperature utilizing the steams latent heat, some may tolerate some degree of water logging and thereby also use the sensible heat contained in the condensate as it cools down to 100șC. The type of trap selected must therefore be matched to its intended use if the most effective energy use of the steam system is to be realized. 

BASIC PRINCIPLES OF STEAM TRAP OPERATION 

The many different types of steam traps manufactured today all operate by sensing the difference between steam (a hot gas) and condensate (a cooler liquid) using one or more of three basic principles, each design has advantages and limitations which must be considered when selecting a steam trap. 

Density Operated—(Mechanical traps) 

(1)  Float

(2)  Inverted Bucket/Open top Bucket. 

Temperature Operated—(Thermostatic Traps) 

(1)  Balanced-Pressure Thermostatic

(2)  Thermostatic

a.      Liquid Expansion

b.      Bimetal Expansion 

Kinetic Energy Operated—(Disc and Orifice Traps) 

(1)  Thermo –Dynamic Disc Traps

(2)  Impulse or Orifice 

DENSITY OPERATED TRAPS 

Density operated traps are purely mechanical in operation. They use some type of float Bucket to determine the condensate level in the body and thereby operate a valve mechanism. Each float Bucket mechanism combination has a fixed mechanical advantage requiring the valve seat orifice to be matched to a maximum operating pressure. 

Float and Thermostatic Traps: 

Ball-float and thermostatic traps have much to recommend them wherever possible. Their valve seat is always under water preventing any steam loss. The discharge is continuous and modulates with the condensing rate, and it is unaffected by any change in inlet pressure. A separate thermostatic air vent independently purges air giving a fast start-up and discharges in parallel with the main valve seat unaffecting its operation. 

Operation: 

Air entering the trap is immediately discharged through a high capacity auxiliary air vent. Condensate causes the ball float to rise and place the modulating discharge valve in a position that will pass the condensate continuously as it enters the trap. The condensate level in the trap body is maintained above the discharge valve to provide a positive seal against the loss of steam. 

Advantages: 

         Discharges condensate continuously as rapidly as it forms.

         High air venting capacity through auxiliary balanced pressure air vent, which is self-adjusting for varying steam pressures.

         High thermal efficiency at both light and heavy loads.

         Continuous modulating discharge does not create pressure disturbances, which may cause erratic control in air hearing coils, shell-and-tube exchangers, etc

         Steam lock release (S.L.R.) facility available.

         Inline inlet and outlet facility, easy installation at low cost.

         Robust, fair resistance to water hammer.

         Fail close, so no wastage of steam energy.

         Unaffected by changes in inlet pressure. 

Limitations: 

         Cannot be used where trap is fitted with air vent bellows, & degree of super       heat > 100șC

         Applications subjected to freezing must be protected with insulation & SLR.

         Water hammer can damage float and air vent bellows. 

Typical application: 

Air unit heaters, Hot water heaters, Heat exchangers, converters, Reboilers, Jacketed Pans etc. 

Bucket Traps: 

Inverted bucket traps have been in existence for many years, and an inexpensive initial cost helps keep them popular although in every application superior results can be obtained with another type of trap. They consume a small amount of steam in operation and can blow fully open if they lose their prime due to over sizing or a rapid drop in inlet pressure. Their discharge is intermittent, not continuous.

Open bucket traps are variation, which are no longer generally accepted and are marketed today as an open-float and thermostatic trap. This type of trap has no mechanical advantage and therefore requires a large heavy bucket, which collects dirt and scale. They have large heavy bodies with limited operating pressures and applications. 

Operation: 

The trap body is normally filled with condensate to maintain a seal around the inverted bucket, which serves as a float to operate the discharge valve. Live steam entering the bucket floats it to close the valve. During the closed period, condensate collects in the piping at the inlet side of the valve until steam floating the bucket leaks through a small hole in the top of the bucket and permits the bucket to drop and open the valve. The condensate is discharged, followed by steam, which is required to actuate the float mechanism. Air can pass through the small hole at the top of the bucket. Some inverted bucket traps are fitted with an auxiliary bimetal air vent.

 Advantages: 

         Fair resistance to water hammer.

         Can be made for very high pressures. 

Limitations: 

         Low thermal efficiency under varying loads and pressures, some steam loss for      operation.

         Must maintain water seal to avoid continuous discharge of steam.

         Must be protected from freezing.

         Cannot discharge condensate continuously as rapidly as it forms.

         Bleed hole in bucket has very limited air-venting capacity.

         Bimetal auxiliary air vent must be factory set at low temperature—not                   self-adjusting. 

Typical applications: 

High pressure indoor steam main drips and submerged heating coils. 

TEMPERATURE OPERATED TRAPS 

Traps of this type all sense the difference between steam temperature and cooler condensate temperature utilizing an expanding bellows or bimetal strip to operate a valve head. They usually discharge condensate below steam temperature and some models require a collecting leg before the trap to allow for condensate cooling. 

Balanced-Pressure Thermostatic Traps: 

Balanced-Pressure Thermostatic traps have a full-open “Blast-type” discharge which permits fast startups and high air venting capacity but limits their operation to pressure up to 17 BAR. Their valve seat is wide-open when cold making them freeze-proof in outdoor use. The bellows material may be brass, phosphor bronze, or corrosion resistant monel or stainless steel.

Temperature-sensitive Balanced Pressure Thermostatic Trap is sensitive to pressure variation, and adjusts to discharge condensate at pre-determined temperature differential at all pressures within its range. 

Liquid Expansion Traps: 

The liquid expansion type traps sense the condensate temperature downstream from the seat, which can be a maximum of 100șC and are therefore highly thermal efficient because they use the sensible heat contained in the condensate. But due to sluggish response and non-self adjusting characteristic, its use is limited. 

Bimetallic Traps: 

Bimetal operated traps sub-cool the condensate well below saturated steam temperature before it is discharged. This means that partial flooding of the steam lines occurs ahead of the trap and care must be taken when they are used to drain steam mains or water hammer damage may result. However, applications such as steam tracing lines, which can tolerate some back up, can realize energy saving by reclaiming some of the sensible heat content of the condensate as it cools down. The bimetals react rather slowly to load changes and may require adjustment for tight shut off due to metal fatigue and operating condition changes. This is not necessary with the Spirax Sarco SM21 trap because its patented “Multi-cross” bimetal element can self-adjust to operating pressure changes.

Bimetallic traps perform well on superheated steam applications when installed with an adequate cooling leg ahead of them. 

BALANCED-PRESSURE THERMOSTATIC TRAPS

 Operations: 

The trap is operated by a flexible thermostatic bellows filled with a fluid, which when heated or cooled, evaporates or condenses. Internal pressure changes expand or contract the bellows and positions the attached valve head. On start up, the cold bellows is contracted, and the wide-open valve remains open to discharge air and sub cooled condensate. When steam reaches the trap, the bellows expands and closes the trap. When condensate surrounding the bellows cools to approximately 8ș to 20șC below steam temperature (depending on the filling), the trap opens, to discharge condensate.

 Advantages:

          High air venting capacity for fast start-up.

         Large capacity in small size.

         Self-adjusting; will operate without adjustments at all pressures within its range.

         Will not freeze if given free discharge.

         Uses same valve sizes for all pressures within its operating range.

         Minimum parts.

         Operates in vacuum systems.

 Typical applications: 

Steam radiators, Low and medium pressure submerged heating coils, sterilizers, and steam tracer. 

 Limitations: 

         Most are not suitable for highly superheated steam.

         Limited resistance to water hammer ad corrosion depending on bellows construction.

         Not suitable for applications in which condensate must be discharged as fast as it is formed. Condensate must cool before it can be discharged.

         Blast discharge at higher pressures with most models

         May fail closed. 

LIQUID EXPANSION THERMOSTATIC TRAP

 Operation: 

Air and condensate are discharged on start-up until he condensate reaches a predetermined temperature below 100șC. The liquid-filled thermostatic element then throttles the valve to maintain the pre-set condensate discharge temperature. 

Advantages: 

         Withstands water hammer

         Very high thermal efficiency (utilizes sensible heat along with latent heat of steam)

         Low-temperature discharge eliminates flash steam around operating stations.

         Will not freeze when given discharge

         Generally fails in an open position. 

Typical applications: 

Steam tracing lines, Storage tank coils, and open tank heating coils. 

Limitations: 

         Limited to applications such as storage tanks and some tracer lines where condensate can be held back and sub cooled before discharge.

         Corrosive condensates can attack bronze bellows in thermostatic element.

         Not self-adjusting to pressure changes.

         Must have an open discharge outlet. 

BIMETALLIC THERMOSTATIC TRAPS: 

Operation: 

Air and condensate are discharged on start-up until the condensate reaches a predetermined temperature. The bimetal thermostatic element then throttles to maintain the preset sub-cooled condensate discharge temperature and some flooding of the steam line may occur. Tight shutoff depends on accurate adjustment of the valve stem length unless the bimetals are designed to self-adjust to operating conditions. 

Advantages: 

         Withstands water hammer.

         Very high thermal efficiency when set to discharge at low temperature.

         Low-temperature discharge eliminates flash steam around operating stations.

         Some designs will not freeze when given free discharge.

         Perform well on superheated steam applications. 

Typical applications: 

Steam tracing lines that can tolerate partial flooding, storage tank coils. 

Limitations: 

         Inconsistent operation and slower response

         Limited to applications in which condensate can be held back and sub cooled before being discharged.

         Bimetal characteristics may change after being in use requiring service.

         Not self-adjusting to inlet pressure changes. 

KINETIC ENERGY OPERATED TRAPS 

Traps that operate on kinetic energy use the velocity and phase change of pressurized condensate flashing to lower pressure steam as the moving force to operate a valve. They appear to be of more simple construction than any other type of trap but successful operation depends on a higher degree of engineering design and skill than with any other type of trap. 

Disc Traps: 

Disc traps are the most widely used steam traps today largely due to their small size, wide pressure range, one moving part, and resistance to water hammer and corrosion. Because operation of each model depends on the manufacturers seat and disc design, results obtained by the user may vary widely. Many are prone to air-binding on start-up, operate below steam temperature causing water logging, have a relatively short life due to soft seat and disc materials, and contain a bleed slot which causes rapid cycling and steam loss.

The Spirax Thermo-Dynamicź series of traps have been carefully engineered to overcome all of the above problems. They discharge condensate and entrapped air at steam temperature regardless of the inlet pressure—with no steam loss. Their hardened seat and disc insure a long service life, even with high backpressures, and the audible cycle provides an instant test of its operation. 

Impulse Traps:

The first operational design was the impulse trap in which two orifices in series create a changing intermediate pressure to operate a piston-type of valve head. It operates far below steam temperature backing up considerable amounts of condensate before opening. Because it requires a constant bleed for operation, it can waste steam on light loads and its use has largely been displaced by disc traps. 

Fixed Orifice (Trap): 

As hot condensate passes through an orifice, the amount of downstream flash produces a backpressure, which increases with the condensate temperature and thereby throttles somewhat the flow rate through the orifice. Orifice devices, which are sold to replace steam traps, must therefore be accurately sized to a constant load and inlet pressure, which prohibits any variation between start-up and operating conditions, or any safety factor. Since orifice devices cannot automatically adjust to any changes they should not be confused with, or used in place of a steam trap. Their small size makes them prone to both erosion and clogging, and light loads and pressure changes can produce live steam leakage. 

THERMO-DYNAMIC DISC TRAPS 

Operations: 

Condensate and air raise the disc and flow freely through the trap. When steam reaches the trap, the velocity under the disc is instantly increased, and recompression above the disc snaps it onto its seats to give a tight shut-off. Heat loss from the small control chamber, which is filled with a steam/condensate mixture, causes the chamber pressure to decrease to a point at which the valve disc opens again to discharge condensate. 

Advantages: 

         Compact, lightweight.

         All stainless steel construction for good corrosion resistance.

         High resistance to water hammer.

         Long life due to hardened seat and disc.

         Only one moving part.

         One trap for all pressures from 0.6 to 100 BAR

         Efficient operation under varying loads and pressures.

         Fast response to changing loads.

         Discharges condensate at steam temperature, to prevent water logging. 

Typical applications: 

Steam main drips, High pressure and superheat, Steam tracer lines, Unit heaters. 

Limitations: 

         Not suitable for pressures below 0.6 BAR

         Some models limited to 50% backpressures, others suitable for 80% backpressures.

         Not recommended for low-pressure applications with temperature control valves. 

IMPULSE TRAPS 

Operation: 

Uses two orifices in series to provide a pressure impulse to operate the discharge valve. When relatively cool condensate reaches the trap, it passes through the two orifices in series without creating sufficient pressure in the control chamber to close the main valve. Condensate continues to flow until it reaches a temperature approximately 17șC below steam temperature, at which point pressure in the control chamber can close the trap. As the condensate is held back and is cooled, the trap again opens, and the cycle is repeated. Under light loads, live steam will reach the trap and be discharged through the bleed orifices. 

Advantages: 

         Small, light weight.

         All stainless steel construction.

         Good resistance to superheat and water hammer. 

Typical applications:  

    None recommended 

Limitations: 

         Continuous bleed orifices will waste steam on light loads.

         Close-fitting valve parts are subject to sticking.

         Condensate held back can cause water logging.

         Condensate held back may contribute to corrosion and water hammer.

         Should not be used where backpressure will exceed 30% of inlet pressure.

 

 

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